You shouldn't have any trouble getting the permits you need for that day if you are able to get online the day before. Permit use has been considerably low this summer due to all the scalpers. From the NPS website:

Updated July 26, 2011 Initial hiker counts for this season indicate that there are numerous no shows among Half Dome permit holders. In an effort to make up for these no shows, the National Park Service (NPS) will manually release additional Half Dome permits each day, at 7 am PDT on the day before the permit date. For instance, at 7 am on Friday, additional permits will become available online (recommended) and through the call center at Recreation.gov for use on Saturday. This will continue throughout the summer until further notice. These permits are taken in a matter of seconds, but it's worth continuing to check because some transactions are not completed, resulting in a few permits becoming available as late as 7:30. Additionally, some permits are canceled every day (and can be canceled and re-reserved until midnight the day before the hiking date).

NPS will initially release an additional 50 Half Dome permits each day and then adjust these numbers, either up or down, throughout the season based upon ongoing hiker counts.These additional permits will have $1.50 processing fee and be limited to purchases of four at a time. Unlike the earlier Half Dome permits, these are non transferable. To counter the illegal resale of Half Dome permits, the group leader, whose name is recorded at the time of transaction, must accompany his or her group on their Half Dome hike. Once the permit transaction is completed, the group leader’s name cannot be changed.

The day-before-release method was chosen to counter both illegal resale of permits and speculative buying by the general public. While purchasing a permit the day before does not allow as much advance notice as many people may wish, it should put permits in the hands of hikers who will use them.